Grease Pencil Wont Convert into Bezier curves in Blender V2.80 beta
I am not familiar with that tutorial but this is how I do it in Blender V2.80 beta, a bit different to V2.79b. 
At the time of writing this I could not find shortcuts for these functions.


Once you finished with the drawings, press F3 on your keyboard.
The search box will pop up and type in Convert Grease Pencil and click the Gpencil: Convert Grease Pencil option, see image below. You may see more selection in the list





The below menu will pop up and select the one you need.





The Bezier is created which you can see in the Outliner. The below image is in Edit mode showing the Bezier edit points, which I edited a bit to move it above the object and curved it to make it visible. I created an offset 3D tube extrusion here from the Bezier, that is what you see in green next to the Bezier.